How electromagnetic relays work and why they are needed

Electromagnetic relays: principle of operation, structure, and applications

Electromagnetic relays (sometimes simply referred to as electric relays) are key components in electronic and electrical systems used for controlling and switching electrical circuits. Despite the emergence of solid-state (semiconductor) relays, electromagnetic relays are still widely used due to their reliability, simplicity, and ability to work with high currents and voltages. In this article, we will discuss:

1. The structure and principle of operation of an electromagnetic relay.
2. The applications of electromagnetic relays in various fields.
3. Their advantages and disadvantages, as well as typical areas of application.

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The structure of the electromagnet in a relay

To understand the principle of operation of a relay, it is first worth considering the basics of the structure of an electromagnet. An electromagnet is a coil of wire wound around a ferromagnetic core (for example, made of steel). When an electric current passes through the coil, a magnetic field is created that attracts or repels the moving part of the relay – the armature or another mechanical element.

Main elements of an electromagnetic relay

1. Coil (solenoid)

   • This is wire wound around a core. When current flows through the coil, it creates a magnetic field.
   • The winding wire has a certain resistance, calculated for the operating voltage (for example, 5 V, 12 V, 24 V, 220 V, etc.).

2. Armature (moving element)

   • Located next to the coil and serves as a “lock” that changes its position under the influence of the magnetic field.
   • The armature can be attached to a pivot lever or positioned in such a way that it closes or opens contacts.

3. Contact block

   • Consists of stationary and moving contacts (NO/NC — Normally Open/Normally Closed).
   • When the armature operates, the contacts either close or open, resulting in switching the electrical circuit.

4. Return spring

   • Ensures the return of the armature to its original position when the coil is de-energized.
   • In the relaxed state, the contacts can be either open or closed, depending on how the relay is designed.

The principle of operation of an electromagnetic relay

1. Power supply to the coil
   When power (electric current) is supplied to the coil, a magnetic field is created.

2. Attraction of the moving part
   This magnetic field attracts or repels the armature (the moving part). The result is mechanical movement that changes the state of the contacts (open/closed).

3. Switching of contacts
   Due to the mechanical movement, the contacts in the contact block are either connected or disconnected.

   • If the contact is NO (Normally Open) — it is usually open but becomes closed when current is supplied to the coil.
   • If the contact is NC (Normally Closed) — it is usually closed but opens when current is supplied.

4. Control of the external circuit
   By changing the state of the contacts, it is possible to close or open external electrical circuits. Thus, a small current in the coil can control a large current in another circuit.

5. Breaking the magnetic field
   When the power to the coil is cut off, the magnetic field disappears. The spring returns the armature to its initial position, and the contacts return to their original state.

This action scheme demonstrates the principle of operation of an electromagnetic relay, which underlies most relay designs. It is relatively simple, reliable, and versatile.

Applications of electromagnetic relays: main areas of use

1. Automation and control

• Control of motors and mechanisms: In industrial settings, relays are used for switching electric motors, pumps, fans, and other large consumers that cannot be controlled directly from low-current electronic devices.
• Lighting systems: Used for automatically turning on/off large groups of lamps or floodlights.
• Heating and air conditioning systems: Allow control of boilers, convectors, ventilation systems, etc., ensuring the desired temperature.

2. Electronics and microcontrollers

• Input/output expansion: In microcontroller systems or PLCs (programmable logic controllers), relays serve to switch large currents and voltages compared to weak signals from control electronics.
• Galvanic isolation: Electromagnetic relays provide clear electrical isolation between the control circuit and the load circuit, which is important for safety and protection against interference.

3. Telecommunications and communication

• Switching communication lines: In telephone exchanges and data transmission systems, relays are used for switching voice and digital lines.
• Protection and redundancy: When it is necessary to switch the signal from one antenna to another or from one line to another, relays can serve as a reliable switching mechanism.

4. Energy and electrical networks

• Overload protection: In distribution panels, relays can operate during overloads or short circuits, disconnecting the circuit.
• Switching between power sources: In uninterruptible power systems (for example, switching between the city grid and a generator), electromagnetic relays are used for reliable switching.
• Automatic backup: When power is lost in one circuit, backup power can be automatically activated.

5. Household appliances

• Control of individual units: Refrigerators, washing machines, air conditioners, boilers, and other appliances often use relays to control large currents of heating elements, compressors, and other powerful units.
• Protection functions: Many devices are equipped with relays to prevent emergency operating modes (overheating protection, voltage spikes, etc.).

Advantages and disadvantages of electromagnetic relays

Advantages

1. Simple design and reliability: The mechanical principle of operation is well studied and has been improved over decades.
2. High switching capacity: Can switch relatively large currents and voltages.
3. Electrical isolation: Relays separate the control circuit from the load circuit, reducing the risk of damage to electronic components.
4. Versatility: Suitable for many different applications – from industry to household appliances.

Disadvantages

1. Mechanical wear: Over time, contacts may wear out or oxidize, especially under high currents or inductive loads.
2. Limited switching speed: The mechanical nature of relays does not allow for very high switching frequencies.
3. Power consumption of the coil: To keep the contacts in the working state, the coil must be continuously powered.
4. Loud noise: A characteristic clicking sound may occur during operation and disconnection.

Types and classification of electromagnetic relays

1. By type of contacts

   • SPST (Single Pole Single Throw) — one pole, one contact.
   • SPDT (Single Pole Double Throw) — one pole, two directions (NO/NC).
   • DPDT (Double Pole Double Throw) — two poles, each with two directions, etc.

2. By operating voltage of the coil

   • Low voltage (3 V, 5 V, 9 V, 12 V, 24 V) – common in electronics and automation.
   • High voltage (110 V, 220 V, etc.) – used in industrial and household networks.

3. By purpose

   • General purpose (General Purpose).
   • Time relays (with built-in timers).
   • Protective (for detecting overloads, short circuits).
   • Contactor (a type of powerful electromagnetic relay for high currents).

4. By number of operating cycles (resource)

   • Standard relays with medium resource (tens or hundreds of thousands of operations).
   • Special industrial relays with increased resource (millions of operations).

FAQ about electromagnetic relays

1. What is the principle of operation of an electromagnetic relay?

The principle of operation is based on the creation of a magnetic field in the coil. When current flows through the coil, it attracts or repels the armature, which mechanically closes or opens the electrical contacts, allowing or stopping the flow of current in the external circuit.

2. How does an electromagnetic relay differ from a solid-state relay?

An electromagnetic relay has mechanical contacts, so it produces a characteristic clicking sound during operation and is subject to mechanical wear. A solid-state relay (SSR) has no mechanical contacts and switches the circuit using semiconductor components. SSRs are faster in operation, quieter, but can be more expensive and sometimes require additional heat sinks for heat dissipation.

3. What are electromagnetic relays used for?

They are used for controlling large electrical loads, automating industrial processes, switching communication lines, providing overload protection, and in many household appliances (air conditioners, washing machines, boilers, etc.).

4. What is the structure of the electromagnet in a relay?

The structure consists of a coil of wire wound around a magnetic core and a moving part (armature) that is attracted to the core under the influence of the electromagnetic field. This mechanical movement allows the contact to close or open the electrical circuit.

5. How to choose an electromagnetic relay?

Pay attention to:

• Coil supply voltage (5 V, 12 V, 24 V, 220 V, etc.).
• Maximum current and voltage of the contacts that the relay must switch.
• Number and type of contacts (NO/NC, SPDT, DPDT, etc.).
• Operating environment (temperature, humidity, possible mechanical loads).
• Operating resource and requirements for switching speed.

6. Can an electromagnetic relay be replaced with an electronic (solid-state) relay?

Yes, in many cases it can, but consider:

• Cost and compatibility with operating voltage and current.
• The need for cooling heat sinks for SSRs at high power.
• The advantages and disadvantages of each option (electromagnetic relays better isolate circuits, SSRs can heat up and cause additional energy costs).

Conclusion

Electromagnetic relays are one of the fundamental technologies that enable the automation and control of complex electrical circuits. Their principle of operation is based on the creation of a magnetic field in the coil, which mechanically moves the contacts, closing or opening the external circuit. Due to their simplicity, reliability, and ability to switch large currents, such relays are widely used in industrial systems, telecommunications, household devices, and many other fields.

Understanding the structure of the electromagnet, the mechanism of contact switching, and the features of relay design allows for the correct selection of the required type of device and ensures long-term and safe operation of electrical systems. Considering the advantages and disadvantages will help make an informed decision about transitioning to other technologies (such as solid-state relays) or continuing to use traditional electromagnetic relays.